Rotate end face of Cylinder keeping cylinder diameter
You're probably looking for the Shear tool, shortcut AltCtrlShiftS (mh yes, you'd better not have lost a finger), it will make the selection slide on the X or Y (it's View orientation by default), then press 1 or -1 to get a 45° or -45° angle (the value is an Offset, not an angle, as you are not doing a rotation, you are sliding the vertices on an axis). Open the Operator box to enter or modify the values manually: